This renewal proposal will provide information on the factors which control reaction rates and reaction specificity in protein-protein electron transfer reactions. Such reactions are centrally important in controlling biological energy flow in the electron transport/oxidative phosphorylation system, and also play an important role in many metabolic reactions. We will monitor the kinetics of reactions in the cytc/cyt c peroxidase complex, the cyt b5: hemoglobin (myoglobin) complexes, and the cytc:cyt c1 complex, using a variety of methods developed in our lab with a particular focus on photochemical electron transfer. Concomittantly, structural studies will be pursued to learn about the nature of binding and recognition in these electron transfer protein complexes, and to probe the dynamic behavior of the structure of such protein complexes. Two novel methods have been developed for such studies, focusing on NMR and energy transfer techniques, respectively.